Rotary machines are used to transfer energy between a flow path for working medium gases and rotating elements inside the machine. There are many examples of such machines in widely disparate fields of endeavor.
FIG. 1 shows a side elevation view of the turbofan engine 10 having an axis of rotation Ar. It is one example of a rotary machine of the gas turbine engine type. The turbofan engine is widely used for powering commercial aircraft and military aircraft.
The turbofan engine 10 has a compression section 12, a combustion section 14 and a turbine section 16. The compression section has an annular (core) flowpath 18 for working medium gases. The flowpath leads to the combustion section and thence to the turbine section. In addition, the compression section has an annular bypass flowpath 22 for working medium gases which conducts an annulus of flow around the core flowpath. The flow rate through the bypass duct can be many times the flow rate through the core flowpath 18. In typical commercial turbofan engines, the flow is five (5) times or greater the flow through the core section of the engine.
The core flow path 18 extends through the engine inwardly of the bypass flowpath 22. As the working medium gases are flowed through the engine, the gases are compressed in the compression section 12. The compressed gases are burned with fuel in the combustion section 14 to add energy to gases and expanded through the turbine section 16 to produce power. As the gases are flowed through the turbine section, rotating elements (not shown) receive energy from the working medium gases. The energy is transferred to the compression section by compressing the incoming gases in both the core and bypass flowpaths. A portion of the energy from the turbine section 16 drives large masses of air through the bypass flowpath 22, usually without adding energy to the gases by burning fuel with the gases. Thus, the gases produce useful thrust as they exit the engine at the rear of the engine and at the rear of the bypass duct.
FIG. 2 is a side elevation view of the engine 10 shown in FIG. 1. The engine is partially broken away to show a portion of the interior of the compression section 28. The engine has a low pressure rotor assembly 24 and a high pressure rotor assembly (not shown). The rotor assemblies extend axially through the engine for transferring energy from the turbine section 16 to the compression section 12. The working medium flow path 18 extends through the rotor assemblies. A stator assembly 26 bounds the flowpath and directs the gases as the gases pass through the stages of the rotor assembles.
The compression section includes a first, low pressure compressor 28. The turbine section 16 includes a low pressure turbine 30. The low pressure turbine is the device used to extract energy from the working medium gases. A shaft 32 connects the turbine section to the low pressure rotor assembly 24 in the low pressure compressor. The shaft is typically called the low shaft. A bearing 34 supports the shaft. Energy is transferred via the rotatable low shaft 32 to the low pressure compressor. The shaft drives the low pressure compressor about the axis of rotation Ar at over three thousand revolutions per minute to transfer energy from the low pressure turbine to the low pressure compressor.
The compression section 12 also includes a high pressure compressor 36. The high pressure compressor receives working medium gases from the exit of the low pressure compressor. The high pressure compressor is connected by a second (high) shaft (not shown) to a high pressure turbine. The high shaft is disposed outwardly of the shaft 32 for the low pressure compressor 28. The high pressure compressor is driven by a high pressure turbine 38 downstream of the combustion section 14. The hot working medium gases are then discharged to the low pressure turbine 30 and drive the low pressure turbine about the axis of rotation Ar.
The low pressure compressor 28 is often referred to as the fan-low compressor. Another example of a fan-low compressor is shown the U.S. Pat. No. 4,199,295 issued to Raffy et al. entitled "Method and Device for Reducing the Noise of Turbo Machines." In Raffy and as shown in FIG. 2, the fan-low compressor has a relatively massive fan rotor disk 42. A plurality of relatively massive fan rotor blades 44 extend radially outwardly from the fan rotor disk across the core flowpath 18 and across the by-pass flowpath 22.
FIG. 2A illustrates the relationship during assembly of the engine of the two main subassemblies: a first subassembly of the fan-low compressor with a fan blade 44 installed and another fan blade being installed; and, a second subassembly that comprises the rest of the engine. The fan rotor blades 44 are axially inserted into the fan rotor disk as one of the last steps of assembling the engine. FIG. 2A shows the engine during the method of assembly as discussed below with at least one fan blade 44 installed and with the next fan blade moving on its path of insertion into the rotor disk.
Each fan blade 44 has a root or dovetail 46 which engages a corresponding slot 48 in the fan rotor disk. Alternatively, the fan blade might be pinned to the rotor disk. The low pressure compressor also includes a drum rotor 50 which is part of the low pressure rotor assembly 24. The drum rotor is so called because of its drum-like shape. The drum rotor extends rearwardly from the fan rotor disk.
As shown in FIG. 3, the drum rotor has dovetail attachment members 52. The members adapt the rotor to receive rotor elements such as a plurality of arrays of rotor blades as represented by the rotor blades 54, 56, 58, 62, 64, and 66. The stator assembly 26 has an interior casing or outer case 68 which extends circumferentially about the rotor assembly. The outer case includes a flow path wall 69 for the bypass flowpath. The rotor blades extend radially outwardly across the working medium flow path 18. Each rotor blade has a tip, 30 as represented by the tips 72, 74, 76, 78, 8284. An outer air seal 85 has outer air seal lands 86 which extend circumferentially about the outer case. The outer seal lands are disposed radially outwardly of the arrays of rotor blades to block the loss of working medium gases from the flowpath. These seal lands, generally called "rubstrips", are in close proximity to the rotor assembly 24. A plurality of arrays of stator vanes, as represented by the stator vanes 92, 94, 96, 98, 102 and 104 extend radially inwardly from the outer case into at least close proximity with the drum rotor. Each stator vane has a tip, as represented by the tip 106.
An inner air seal 108 is disposed between the stator vanes 92-104 and the drum rotor 50. Each inner air seal 108 has a seal land 112 which extends circumferentially about the tips 106 of the stator vanes. The seal land is disposed in at least close proximity to the drum rotor. The drum rotor is adapted by rotor elements, as represented by the knife edge seal elements 114, which extend outwardly and cooperate with the seal land to form the inner air seal. The knife edge seal elements have a greater height than width and are relatively thin. The knife edge elements cut into the seal land under operative conditions as the knife edge elements move radially outwardly under operative conditions. An example of such a construction is shown in U.S. Pat. No. 4,257,753 issued to Bradley et al. entitled "Gas Turbine Engine Seal and Method for Making a Gas Turbine Engine Seal." The seal land in Bradley has a thin film surface layer that is resistant to erosion and provides a small amount of wear to the knife edge element. It may be formed of metallic fibers and a silicone based resin.
Another type of material for the seal land 112 is an elastomeric material such as room temperature vulcanizing rubber. One satisfactory material for the inner air seal land is silicone rubber available as DC93-18 silicone rubber from the Dow Corning Corporation 2200 W Salzburg Rd, Auburn, Mich. 48611. A satisfactory material for the outer air seal land 86 (rubstrip) is available as Dow Corning 3-6891 silicone rubber available from Dow Corning Corporation, Midland, Mich. Each silicone rubber is abradable and accepts rubbing contact with rotating structure without destruction.
An assembly clearance and an operative clearance (clearance under operative conditions) are provided between the rotor 24 assembly and stator assembly 26. Examples are the clearances between the rotor blade tips 72-84 and the outer air seal lands 86; between the knife edge elements 114 and the inner air seal land 112 of the stator vanes 92-104; and, between other locations in the engine where rubbing contact might take place between rotating parts and stationary parts in the low pressure compressor and the low pressure turbine.
The assembly clearance provides a radial distance between the rotor elements rotor (blade, knife edge) and the stator assembly to take into account radial tolerances on the rotor disk 42 or drum rotor 50, the blade 44, stator vanes 92-104, and the seal lands 86, 112. The assembly clearance is necessary to permit initial inspection of the assembly by turning (rotating) the assembly about the axis Ar by hand or at very slow speeds with low force. This inspection ensures that a destructive interference does not occur at some location during normal operations of the engine at high speeds. Such interference might occur between parts of the low pressure compressor 28, between parts of the low pressure turbine 30 and between the low shaft 32 that connects them and other parts of the engine. In addition, the clearance is helpful in assembling the fan rotor blades 44 to the fan rotor disk 42.
During assembly of the gas turbine engine 10, the fan blades 44 are axially inserted into the rotor disk 42. These are inserted one at a time. The rotor disk is turned by hand, bringing the slot 48 receiving the rotor blade to a location where a worker can insert the fan rotor blade while standing in front of the engine or on a small step ladder. However, if the rotor assembly binds, the worker must climb a taller ladder and maintain his balance while maneuvering the very heavy fan blade (sometimes weighing in excess of twenty pounds) into some of the higher oriented slots. As a result, workers will try to force the rotor to turn or request that engine be disassembled and reassembled with more clearance. Sometimes a torque in excess of one thousand foot pounds force (1000 ft-lbf) is required to turn the rotor assembly during assembly of the engine. Such a torque may bend the delicate tips 72-84 of the rotor blades 54-66. Accordingly, a too tight nominal clearance or tight minimum clearance dimension might cause a rotor blade to contact a rubstrip and cause bending of the tip of the rotor blade as attempts are made to rotate the compressor by hand. Too tight a clearance might also decrease engine performance by causing the rotor blades to rip out a rub strip under operative conditions, liberating material which might impact downstream components.
These assembly clearances, if too large, may adversely effect the efficiency of the engine particularly the efficiency of the compressor 28. If too large, the clearance at operative conditions may not close at the cruise condition leaving a gap. The gap of concern is between the rotor element and the adjacent surface of the seal land before and after the rotor element. A gap caused by a rub of the rotor element surprisingly has a small effect on aerodynamic performance. However, a gap with respect to the adjacent structure might create a leak path between the rotor assembly 24 and the compressor, such as between the blade tips and the rubstrip and between the knife edges of the drum rotor and the adjacent seal land carried by the tips of the stator vanes. The gap at cruise provides an escape path for the working medium gases around the rotor blades. The gap at cruise is a concern because the engine may spend a significant amount of time at the cruise condition during long flights.
The nominal clearance at assembly is set within a tolerance band (permitted variation) that trades off the need for aerodynamic efficiency against the need for an acceptable assembly clearance; one that facilitates building the rotor assembly by being able to rotate the rotor assembly 24 by hand at very low speeds during fabrication. Accordingly, the nominal clearance at assembly with its tolerance band sets a radial zone of locations for the rotor elements that avoids too tight or too large clearances. For example, for a rotor blade 54, the nominal value of the clearance might be one-hundred and seventy two (172) mils plus or minus twenty-five mils, that is, with a tolerance band from a maximum clearance dimension of one hundred and ninety seven (197) mils to a minimum clearance dimension of one-hundred and forty seven (147) mils. This radial zone of rotor locations may be applied to an element rotating at over three thousand (3000) revolutions per minute at a four foot diameter.
The radial position of the tolerance band at assembly (nominal assembly clearance) must take into account not only assembly and operative aerodynamic considerations, but also the average diameter Dav of the outer case (e.g. outer seal land 86) at a particular axial location. Typically, the stacking line S for the rotor blades in the assembled condition is used as the axial location at the outer air seal for measuring the average diameter of the seal land. (The stacking line S is the spanwise reference line on which the chordwise extending airfoil sections are disposed perpendicular to the stacking line to define the contour of the rotor blade. The average diameter for the seal land is determined by first measuring the circumference of the seal land at that location and then dividing the circumference by .pi.(Dav=C/.pi.). The value must fall within acceptable limits. There is no factor in the average diameter for any anomalies in the outer case (seal lands 86, 112) which may occur outside these limits as a result of further processing during assembly. These are tolerated because the case is large (often over four feet in diameter) with acceptable limits for the average diameter Dav lying within a range of hundredths of an inch.
The matter is further complicated because drum rotors 50 require further processing which includes an axial extending parting line or split in the outer case. The parting line allows the two halves (or more parts if not cut in half in a longitudinal direction) to be bolted together about the drum rotor to dispose the outer seal lands 86 and the inner seal lands 112 about the rotor elements. Accordingly, the average diameter of the seal land is measured as discussed above (Dav=C/.pi.) prior to cutting the outer case with the rubstrip installed. There are many other approaches for measuring the average diameter Dav at the stacking line. These include using a coordinate measurement machine. The measurement machine measures the diameter at many locations around the circumference at a particular axial location. These measurements are then averaged.
The outer case in many applications is several feet in diameter and may be as thin as one hundred and fifty (150) mils and formed of an aluminum alloy such as Aerospace Materials Specification (AMS) 4312. When the case is reassembled from its component parts some additional circumferential anomalies may be introduced in the installed condition by the flanges on the case.
As a result, gas turbine engines are not built with too tight clearances because the rotor, such as a drum rotor 50, cannot be rotated at assembly. As the prototype engines are built, the rotor is rotated and often the tolerances are increased. A positive minimum clearance dimension is always provided for those rotor blades 54, 56, 58 that grow radially outwardly and rub against the rubstrip while attempting to set the nominal (average) clearance dimension to arrive at a line on line clearance (zero clearance) at the cruise condition.
The arrays of rotor blades 62, 64, 66 in the rear of the compressor adjacent a rapidly converging flow path are different. It has been observed that one or more of these arrays of rotor blades do not tend to rub at the cruise operative condition. These rotor blades have a positive nominal clearance dimension with a minimum clearance dimension that is zero as measured at the stacking line; and with a maximum clearance dimension that is greater than thirty (30) mils in one application.
The rotor blade 66 may have a tip 84 which extends rearwardly (chordwisely) at about the same angle as the rubstrip. However, it is angled slightly inwardly in the spanwise directions to provide a taper in the event of a rub. A rub, which might occur at an extreme Sea Level Take Off operative condition, would cause the tapered tip to cut a trench in the rubstrip that is tapered rearwardly with decreasing depth. This taper is provided for aerodynamic reasons. As a result, the forward most portion of the rotor blade 66 at the minimum zero tolerance dimension might have an interference fit of about one to two mils (0.001-0.002 inches). This slight interference fit is accommodated somewhat by the rotor blades being able to lean slightly in the circumferential direction as a result of assembly tolerances at the base of the rotor blade where the dove tail engages the rotor 50. Thus, as the rotor is turned by hand, the blade will slide along the rubstrip at the average diameter Dav of the rubstrip.
The knife edge seal projections or elements 114 are a third category. These are provided with a minimum clearance and a nominal clearance that is smaller than the forward rotor blades 54, 56, 58 but insures that the knife edges cut a groove under operative conditions and run in the groove in the cruise operative condition on the seal land that each knife edge engages.
In summary, there is tension between the need to minimize the aerodynamic clearance in the cruise operative condition of the engine and the need to be able to assemble and inspect the low pressure compressor and low pressure turbine assembly. The matter is further complicated in that the outer case, having an average diameter, is not a true circle at any axial location but has anomalies that extend inwardly at some locations. In short, if it is difficult to rotate the rotor by hand, the solution is to open the clearance by increasing the minimum clearance dimension or the nominal clearance dimension to increase the clearance and allow the rotor to turn more freely. However, this is accompanied by a decrease in aerodynamic efficiency.
The above art notwithstanding, scientists and engineers working under the direction of Applicants Assignee are seeking to address the twin needs of being able to rotate the low pressure compressor-low turbine rotor assembly during buildup while maintaining clearances that are acceptable for aerodynamic performance.